Ultrasound-assisted enzyme extraction of total flavonoids from lotus leaf (Nelumbo nucifera Gaertn.) and its antioxidant activity

Lotus leaf ( Nelumbo nucifera Gaertn.), extensively used in culinary and medicinal applications, is abundant in flavonoids and alkaloids with notable antioxidant and anti-inflammatory effects. This study utilized a response surface methodology to refine the extraction process of total flavonoids (TFs) from lotus leaf (LL) and to evaluate TF content across different sources in relation to antioxidant properties. Ethanol concentration, material-liquid ratio, enzyme dosage, enzyme digestion time, enzyme digestion temperature and ultrasonication time were analyzed through single-factor experiments to assess their impact on TF yield. The Box–Behnken design identified optimal extraction conditions. In vitro antioxidant activity of LL was measured via free radical scavenging assays. The optimized extraction parameters included an enzyme concentration of 30 mg/g, an ethanol volume fraction of 40% (v/v), a material-liquid ratio of 1:40 g/mL,a enzyme digestion time of 50 min, enzyme digestion temperature of 38°C and an ultrasonication time of 20 minutes. Extracts produced through ultrasound-assisted enzyme extraction demonstrated superior TF levels compared to conventional ultrasonic extraction and hot water extraction. Compared with the existing extraction methods, this experiment greatly improved the extraction rate of LL and further enriched the antioxidant activities of TF.